1. Field of the Invention
This invention relates to a thermal fuse for fixing on a circuit board, more particularly to a thermal fuse with a resilient coil portion connected to two spring arms for soldering onto contacts of the circuit board.
2. Description of the Related Art
Referring to FIGS. 1 and 2, a thermal fuse disclosed in U.S. Pat. No. 5,600,295 is cut from a spring strip that is bent to form a spring clip 10 which includes two spring arms 11,12 interconnected by an M-shaped bending section 13. The tension-free form of the spring clip 10 shown in FIG. 2 has two solder supports 14,15 to be soldered to the corresponding solder contacts 21,22 (see FIG. 3) of a circuit board. Referring to FIG. 3, pressure is exerted on the bending section 13 from two sides via bending punches A, B so that the spring clip 10 is deformed from the shape indicated by the solid lines 16 to the shape indicated by the broken lines 17. The spring arms 11,12 are thus pre-stressed. When the fuse is triggered in that the solder on the solder contact 21 melts, the spring arm 11 is detached from the circuit board, and the pre-stressed spring arms 11,12 assume their original shape, whereas the bending section 13 remains in the deformed shape, as indicated by the dotted lines 18, thereby protecting the components on the circuit board against damage due to thermal overload.
Alternatively, as shown in FIG. 4, the bending section 13 is deformed in the direction of the arrow C via a bending punch after soldering onto the solder contacts 21,22, so that the pre-stress phase having the shape 17' (indicated in broken lines) arises. When the fuse responses, the solder support 14 moves away from the solder contact 21, and the fuse assumes the shape 18' shown in dotted lines.
However, the fuse made from the spring strip has a relatively large size and is not suited for use on a relatively small circuit board. In addition, there is verdigris formed on the fuse to result in elastic fatigue of the spring clip 10. As such, the spring arm 11 cannot depart from the solder contact 21 rapidly when the solder thereon melts, thereby affecting adversely the efficiency of the fuse.